1. Field of the Invention
The present invention relates to that type of switching power supplies which uses pulse width modulated signals as the drive signals to the even number of power switching devices included therein and more particularly to the drive transformer for use therewith.
2. Description of the Prior Art
Telephone central offices use d-c voltages for a variety of functions. These d-c voltages may by supplied directly from a bank of batteries which are charged by a battery charger or from a power supply which converts the commercially available 60 Hz voltage from the electrical utility into a d-c voltage of proper amplitude thus eliminating the batteries or from a d-c to d-c converter which receives its input voltage from the batteries.
In the past it was common for converters, chargers and eliminators to be designed so that their power circuitry and control circuitry operated at the 60 Hz frequency of the commercially available a-c voltage. In order to reduce the size, cost and objectionable noise that ordinarily are associated with supplies operating at 60 Hz it has now become common for such supplies to operate at a frequency above the audible range. Selection of such a high operating frequency has the desirable characteristic of reducing the size of the supply and any transformers therein which results in a savings of both component costs and supply weight and, in addition, the supply no longer generates audible noise.
Many types of high frequency converters, chargers and eliminators now make use of the well-known pulse width modulation (p.w.m.) technique to provide a p.w.m. signal to drive one or more power switching devices at the relatively high frequency. The p.w.m. technique usually makes use of a sawtooth waveform of predetermined frequency for purposes of generating the drive signal to the power switching devices. Typically the frequency of the sawtooth waveform is in the order of 40 KHz when the supply has two or more alternately conducting power switching devices arranged, for example, in a half bridge or full bridge configuration.
The p.w.m. drive signals are connected to the power switching devices by a drive transformer. It has also been common in such arrangement to include in the drive transformer a means to provide regenerative drive to the power switching device being turned on by its associated drive signal. Regeneration, as is known in the art, aids and reinforces that turnon. Where the supply includes an even number of power switching devices which conduct alternately, it is the practice in the art for each such device to have its own drive transformer. The use of separate drive transformers provides isolation between the alternately conducting power devices. It was thought that such p.w.m. driven, alternately conducting power devices could not share a single drive transformer.
It was then recognized that such devices could share a single drive transformer without any detrimental effect. It was further recognized that the single drive transformer could include a regenerative winding which would also be shared by the alternately conducting power switching devices. It was additionally recognized that such an arrangement would not only not have any detrimental effect on the operation of the power switching devices and by the regenerative winding aid the turnon of the then off device, but would also substantially improve the turnoff of the then conducting device.